1. Field of the Invention
The present invention relates to electronic assemblies for electronic interconnect applications. More particularly, the present invention relates to electronic assemblies which include an elastomeric member made of a cured, room-temperature curable polysiloxane composition. When the assemblies are used to electrically interconnect a first contacting site on a first electronic device to a second contacting site on a second electronic device, the stress-relaxation resistant properties of the elastomer enhance local contact force to maintain a reliable connection. In addition these polysiloxane compositions exhibit exceptional stress-relaxation resistance during high temperature aging.
2. Description of Related Art
Conventional electrical connectors using metal pin and spring beam contacts cannot be easily miniaturized to satisfy the anticipated pin count density for high performance electronic devices. The electrical characteristics of these connectors cannot meet requirements such as propagation delay, risetime degradation, reflection, and crosstalk. The increasing demand for higher speeds and higher I/O contact density in electronic devices has led to the development of new connectors which utilize non-metallic components to produce and maintain contact force.
For example, some connectors are no more than elastomeric matrices loaded with electrically conductive materials. The elastomeric matrix is placed between the contacts on a first electronic device and the contacts on a second electronic device, the devices are pressed together, and the conductive materials provide electrical interconnection. Such connectors are well known, and examples are shown in U.S. Pat. No. 5,049,085 to Reylek, U.S. Pat. No. 4,008,300 to Ponn, U.S. Pat. No. 5,037,312 to Casciotti et al., U.S. Pat. No. 5,275,856 to Calhoun, and U.S. Pat. No. 4,003,621 to Lamp. The connectors described in these patents utilize a wide variety of elastomeric materials, including butadiene-styrene, butadiene-acrylonitrile and butadiene-isobutylene rubbers, chloroprene and polysulfide polymers, polyvinyl chloride, vinyl acetates, polyurethanes and silicone rubbers. The ""621 patent to Lamp states that silicones are preferred, and these materials may be selected from dimethyl, methyl-phenyl, methyl-vinyl or halogenated siloxanes. These silicones may be cured with peroxides or metal salts. The Lamp ""621 patent further states that a useful silicone should not deform under its own weight and should not plastically deform after curing.
In some applications, particularly with flexible circuits, the standard metal pin and metal spring socket contact is replaced with a contact in which electrical interconnection is established by mechanically pressing a first contact pad on the circuit to a second contact pad on the connector, device or other circuit. The pressure connections are normally made with a resilient pressure applicator, such as an elastomeric member. The elastomeric member is compressed to bias at least one of the components to be electrically interconnected toward the other components to hold the contact pads thereof in electrical contact. Examples include U.S. Pat. No. 5,009,607 to Gordon et al., U.S. Pat. No. 5,186,632 to Horton et al., U.S. Pat. No. 5,059,129 to Brodsky et al., U.S. Pat. No. 5,313,368 to Volz et al., U.S. Pat. No. 4,636,018 to Stillie, and U.S. Pat. No. 3,967,162 to Ceresa et al. These patents teach that a wide variety of polymeric materials may be used as the elastomeric member, and silicone rubbers are in many cases preferred. For example, the ""129 patent to Brodsky states that important properties of the elastomeric material include long-term stress retention, low magnitude pressure against the contacts, and resistance to high temperatures, solvents and humidity. The preferred elastomeric material in the ""129 patent is a low compression set polysiloxane (silicone) rubber.
In addition, it is well known that elastomeric compressive members may be used to bias a component against a connector, a circuit, or another device. Examples include U.S. Pat. No. 5,345,364 to Biernath, U.S. Pat. No. 4,867,689 to Redmond et al., and U.S. Pat. No. 4,548,451 to Benarr et al. For example, the ""451 patent to Benarr states that any elastomeric material which maintains a xe2x80x9cuniform compressive forcexe2x80x9d may be used as the compressive member, such as silicone or polyurethane. The ""364 patent to Biernath states that the elastomeric component may comprise rubbers, foams and the like.
Therefore, it is well known to use rubbery materials, particularly silicones, with low compression set as an elastomeric member in an electronic connector. Compression set resistance is defined as the ability of an elastomeric material to recover its pre-stressed shape after removal of the stressing members (ASTM D 395). The compression set resistance is a measure of a dimensional change in an elastomeric material following removal of an applied stress.
However, the principal function of an electrical connector is to maintain electrical interconnection between a first set of contacts on a first device and a second set of contacts on a second device. If reliable electrical interconnection is to be maintained, the force applied by the connector at the contact interface must remain substantially constant, especially when the connector is exposed to an externally applied mechanical force, or to environmental stresses such as heat, humidity, solvents, and the like. If an elastomer is used as a component part of such a connector, the elastomer selected must have the ability to maintain the normal force at the contact interface, which is referred to in the art as the xe2x80x9ccontact force,xe2x80x9d rather than simply maintaining its pre-stressed dimensional shape.
Force-bearing elastomers in electronic components must have stable force-bearing capabilities at high temperatures for long durations of time (e.g., 1000 hours at 125xc2x0 C.). These requirements are dictated by their usage and standardized by standards organizations (See, for example: Military Standard 1344A Test Methods for Electrical Connectors.).
If the resistance at the contact interface is to remain low and the contact force is to remain high, the normal force exerted by the silicone elastomer at the contact interface must remain high following extended exposure to mechanical force and to the environment. Therefore, for electronic connectors, a silicone elastomeric material is needed in which a high percentage of this normal force is retained in the portion of the elastomer adjacent to the contact interface following exposure to mechanical and environmental stress. The proper parameter to measure a silicone elastomer""s suitability for use in electronic connectors is the stress relaxation resistance, which is a measure of the percent of the applied mechanical force retained by the material after exposure to both mechanical stress and the environment.
The references discussed above teach that an elastomer with low compression set, preferably a silicone elastomer, is well suited to maintain electrical interconnection in an electronic device. However, there is no direct correlation that can be established between compression set resistance (a dimensional property) and stress relaxation resistance (a force/pressure property). For example, an elastomer that exhibits 100% initial size recovery (thus, 0% compression set) after aging may require only a fraction of the initial force loading to re-compress the material. The compression set resistance of a silicone elastomeric material is therefore an insufficient measure of its suitability to maintain contact force in an electronic connector application.
In addition to the requirement of excellent stress relaxation resistance, a silicone elastomeric material selected for use in an electronic connector must be easily moldable to a wide variety of highly precise shapes. The silicone must flow easily to adapt to the precise dimensions of the mold. During the curing process, the silicone must retain high dimensional accuracy. Changes in contact normal force may result from dimensional variations, so the electrical interconnection of precision electronic components can be adversely affected by dimensional changes. Some elastomers may also require precise lateral dimensional accuracy in some designs to ensure proper alignment between their conductive regions and the contact pads to be interconnected. The silicone must also be rapidly curable at a low temperature. Extended cure times are unacceptable for commercial production processes and the high temperatures may damage delicate electronic components. In addition, high curing temperatures may adversely affect the dimensional accuracy of the molded material. Further, the curing process must not produce by-products that can damage or corrode delicate electronic components.
At present, no silicone elastomeric material is available which has the above combination of properties. In fact, as noted above, the silicone elastomers which are presently available have excellent compression set resistance, which is of little or no import for electronic connector applications. In addition, it is conventionally taught that a high temperature cure is required to achieve compression set resistance. For example, U.S. Pat. No. 5,219,922, Dow Corning product literature, p. 60, form #10-008F-91, and U.S. Pat. Nos. 5,153,244, 5,219,922, and 5,260,364 suggest that a high temperature cure is required to produce an elastomer with high temperature compression set stability. These patents and publications suggest that the requirement of high temperature force-bearing stability conflicts with the requirement for low temperature cure.
It is also generally understood and practiced that the platinum catalyst concentration should be minimized in silicone elastomer compositions, primarily due to economic considerations (U.S. Pat. No.5,153,244). A lower limit of 0.1 million parts (ppm) by weight platinum metal per the combined weight of all the reactive ingredients is specified, below which the cure does not proceed satisfactorily.
It would be desirable to provide a silicone elastomeric material with the precise combination of properties required for electronic connector applications, such as excellent stress relaxation resistance, low temperature cure, excellent dimensional stability, and an absence of detrimental reaction byproducts. The present invention is based on such a finding.
The present invention is an electronic assembly comprising a force bearing member made of an elastomeric cured silicone composition. The silicone composition used to make the elastomeric member comprises:
a) an addition curable silicone polymer comprising an average of at least 2 unsaturated functional groups, preferably vinyl, per molecule;
b) a crosslinker comprising an average of at least 2 silicon-hydrogen linkages per molecule; and
c) a catalyst, preferably comprising platinum.
The catalyst is present in an amount sufficient to permit curing of the composition in less than about 1 hour at a temperature of about 30xc2x0 C. Preferably, following curing, the composition has a predetermined stress relaxation resistance, preferably at least 75%, as measured according to a modified procedure described in ASTM-395 (measured as percent force retained).
The cured silicone elastomeric composition of the present invention has an improved stress-relaxation resistance compared to conventional silicone elastomers. These properties enable the elastomeric member to maintain a predetermined level of contact force to ensure reliable electrical interconnection for extended periods. The elastomeric member may act as a spring member, a force distributor, and/or a compliant layer in the electronic connector assembly. The silicone composition of the invention cures rapidly at low temperature, retains excellent dimensional stability during the curing process and thereafter, and does not release detrimental by-products during the curing process.
In one embodiment, the present invention provides an electronic connector subassembly which includes an elastomeric member with at least one electronic contacting site adjacent thereto. The first contacting site on the subassembly may be placed in contact with at least a second contacting site on another device or circuit structure, such as, for example, a circuit board, a flexible circuit, or an electronic component. A mechanical force may be placed on the subassembly to bias the elastomeric member and maintain electrical interconnection between the first contacting site and the second contacting site. The elastomeric member is made of the cured silicone composition described above.
In another embodiment, the present invention provides an electronic assembly which comprises a female member with a first contacting site, and a male member with a second contacting site. An elastomeric member made of the cured, silicone composition described above is positioned between the female member and the first contacting site, or between the male member and the second contacting site, or both. When the male member is inserted into the female member, a mechanical force is applied to bias the elastomeric member(s) and maintain a reliable electrical interconnection between the first contacting site and the second contacting site. The elastomeric member(s) acts as a spring member and/or a force distributor in the electronic connector assembly.
In yet another embodiment, the present invention provides an electronic assembly comprising a first substrate such as, for example, a printed circuit board, with a first electronic device mounted on and/or electrically interconnected thereto. The first device, or the first substrate itself, or both, has at least one first electrical contacting site. A second substrate may have a second electronic device mounted on and/or electrically interconnected thereto. The second device, or the second substrate itself, or both, has at least one second contacting site. An elastomeric member made of the cured silicone composition described above may be placed between the first substrate and the first contacting site, or between the second substrate and the second contacting site, or both. A mechanical force is then applied to bias the elastomeric member and maintain an electrical and/or thermal interconnection between the first contacting site on the first substrate or first device and the second contacting site on the second substrate or device. The member(s) made from the cured silicone elastomer acts as at least one of a force distributor, a spring member, a planarity compensator, or a thermal mismatch buffer, and mechanically decouples the electronic devices from the substrate.
In another embodiment, the present invention includes an electronic assembly which comprises a first electronic component with a first contacting site and a second electronic component with a second contacting site. An elastomeric member made of the cured silicone composition described above may then be placed between the first contacting site and the second contacting site. The elastomeric member may be loaded with conductive particles or provided with an array of discrete conductive members to form at least one conductive path between the first contacting site and the second contacting site. When a mechanical force is applied to the assembly to bias the elastomeric member, the elastomeric member again acts as at least one of a force distributor, a spring member, a planarity compensator, or a thermal mismatch buffer to reliably electrically and/or thermally interconnect the first site to the second site.
In each of the above electronic assemblies, the dimensional stability and excellent stress relaxation resistance properties of the elastomeric members made from the cured silicone composition of the invention are used to advantage to provide local force concentration to maintain contact force at the interface between the respective contacting sites and ensure reliable electrical interconnection. The elastomeric members also provide local compliance and adjust for a wide variety of dimensional differences, such as, for example, the height of the contacting sites or defects in the substrates.